The present invention relates to an electron-emitting device comprising a surface layer composed of a material having a negative or substantially negative electron affinity.
A conventional electron-emitting device has a structure in accordance with a hot cathode system (an electron gun system) in which hot electrons are emitted from a solid into a vacuum by heating, to a high temperature, a cathode composed of a refractory metal material such as tungsten (W) which is disposed in spaced opposing relation to an anode. As a recent electron beam source substituting for an electron gun, attention has been focused on an extremely small electron-emitting device of cold cathode type. As typical electron-emitting devices of this type, there have been reported a device of field emission type and a device of pn junction type having a coating of a low-work-function material on the surface thereof.
The electron-emitting device of field emission type comprises a conic emitter portion formed of a refractory metal material such as silicon (Si) or molybdenum (Mo) and a withdrawn electrode disposed in spaced relation to the emitter portion. A voltage is applied to the withdrawn electrode to produce a high electric field ( greater than 1xc3x97109 V/m), which causes the emitter portion to emit electrons. The device of field emission type is advantageous in that it can be scaled down by using a micro-fabrication technique.
On the other hand, the electron-emitting device of pn junction type has a coating layer of a low-work-function material, such as cesium (Cs), on the surface of a p-type semiconductor layer, thereby producing a state with a negative electron affinity. The device is structured such that electrons are emitted into a vacuum from the p-type semiconductor layer with the application of a bias voltage to the device.
The characteristics equally required of the extremely small electron-emitting devices of cold cathode type are: easy emission of electrons with a relatively small operational voltage (e.g., an emitter material has a low electron affinity); easy control of the emitted electron beam; the chemically stable surface of the emitter portion owing to a stable emission property; and excellent wear resistance and excellent heat resistance.
However, the conventional electron-emitting devices mentioned above have the following disadvantages.
The device of field emission type has the disadvantage of a relatively high operational voltage because a high electric field is used to cause electron emission. The device also has the disadvantage of poor controllability of the amount of emitted current because the amount of emitted current is highly dependent on the configuration of the emitter portion and on the surface state and because it is difficult to form a large number of conic emitters with excellent uniformity.
The electron-emitting device of pn junction type using a semiconductor material is disadvantageous in terms of device stability and lifespan because the coating layer made of a low-work-function material, which is indispensable to the surface of the p-type semiconductor layer for achieving easy emission of electrons, is poor in stability.
It is therefore an object of the present invention to provide a novel electron-emitting device of previously unknown structure, which is operable with a low voltage and excellent in emission property, by using a materiel having a negative or substantially negative electron affinity close to zero and by providing means for efficiently supplying electrons to the surface layer serving as an electron-emitting portion.
A first electron-emitting device of the present invention comprises: a semiconductor layer functioning as an electron supplying layer; a surface layer disposed in spaced relation to the semiconductor layer, the surface layer being composed of a material having a negative electron affinity or a positive electron affinity close to zero; a graded composition layer interposed between the semiconductor layer and the surface layer, the graded composition layer having a varying composition such that the electron affinity decreases in a direction from the semiconductor layer toward the surface layer; and a surface electrode disposed on the surface layer, the surface electrode applying a voltage such that electrons are supplied from the semiconductor layer to an outermost surface of the surface layer.
In the arrangement, the graded composition layer as the region in which the electrons move has the electron affinity decreasing with approach toward the surface layer. Accordingly, the electrons are easily emitted from the surface layer having a negative electron affinity into a vacuum by applying a voltage between the semiconductor layer functioning as the electron supplying layer and the surface electrode and thereby supplying the electrons to the surface layer through the graded composition layer. Since a voltage of about 10 V is sufficient as the maximum voltage to be applied, though it varies depending on the device structure, there can be implemented an electron-emitting device operable with a low voltage and capable of emitting the electrons with high efficiency.
In the first electron-emitting device, the electrons can be supplied efficiently from the electron supplying layer to the surface layer by using a non-doped semiconductor material for the graded composition layer.
The first electron-emitting device preferably uses the structure in which at least a part of the graded composition layer has a band gap enlarged nearly continuously from the semiconductor layer to the surface layer. In the structure, the electrons are allowed to move smoothly in the graded composition layer.
In the first electron-emitting device, the surface layer preferably contains aluminum nitride (AlN). The arrangement allows the formation of the device capable of emitting electrons with high efficiency by using the characteristic of negative electron affinity of the AlN surface.
Preferably, the first electron-emitting device further comprises a coating layer disposed on the surface layer, the coating layer being composed of a material different from the material composing the surface layer. The coating layer can control electron emission and protect the surface layer.
In the first electron-emitting device, the coating layer preferably contains a material having a negative electron affinity or a positive electron affinity close to zero. In the arrangement, the property of efficient electron emission can be maintained, while electron emission can be controlled and the surface layer can be protected.
In the first electron-emitting device, the coating layer preferably contains aluminum nitride (AlN).
In the first electron-emitting device, a region including the graded composition layer and the surface layer is preferably composed of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) in which the proportion of Al increases with approach toward the outermost surface. The arrangement allows easy formation of the high-quality graded composition layer and surface layer.
In the first electron-emitting device, the surface electrode is preferably in Schottky contact with the surface layer. In the arrangement, the electrons can be supplied with excellent controllability from the electron supplying layer.
In the first electron-emitting device, the surface electrode preferably has first and second regions, an energy barrier between the surface layer and the second region being larger than an energy barrier between the surface layer and the first region. In the arrangement, the current of electrons can converge on the first region of the surface electrode and the density of the emitted current can be increased.
In the first electron-emitting device, the surface electrode preferably has a pattern for controlling the distribution of emitted electrons. As a result, a current of emitted electrons having a desired distribution can be generated with controllability.
The first electron-emitting device further comprises: an insulator layer overlying a part of the surface electrode; and an external electrode disposed on the insulating layer, the external electrode accelerating and controlling the electrons emitted from the surface layer to the outside. Consequently, there can be achieved an integrated acceleration/control mechanism for the current of emitted electrons.
A second electron-emitting device of the present invention comprises: a semiconductor layer functioning as an electron supplying layer; a surface layer disposed in spaced relation to the semiconductor layer, the surface layer being composed of a material having a negative electron affinity or a positive electron affinity close to zero; a multilevel superlattice layer interposed between the semiconductor layer and the surface layer, the multilevel superlattice layer being composed of a plurality of stacked layers for resonantly transmitting electrons supplied in a direction from the semiconductor layer toward the surface layer; and a surface electrode disposed on the surface layer, the surface electrode applying a voltage such that electrons are supplied from the semiconductor layer to an outermost surface of the surface layer.
In the arrangement, the electrons are efficiently transported from the semiconductor layer to the outermost surface of the surface layer via the subband formed in the multilevel superlattice layer with the application of the voltage to the surface electrode. Consequently, there can be obtained a device which emits electrons with high efficiency by merely applying a low voltage thereto.
In the second electron-emitting device, the multilevel superlattice layer is preferably composed of two mixed crystals each represented by AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61), the two mixed crystals having different component ratios and being alternately stacked in layers. The arrangement allows easy formation of an excellent multilevel superlattice layer.
In the second electron-emitting device also, the surface layer preferably contains aluminum nitride (AlN).
Preferably, the second electron-emitting device also further comprises a coating layer disposed on the surface layer, the coating layer being composed of a material different from the material composing the surface layer.
In the second electron-emitting device also, the coating layer preferably contains a material having a negative electron affinity or a positive electron affinity close to zero.
In the second electron-emitting device also, the coating layer preferably contains aluminum nitride (AlN).
A third electron-emitting device of the present invention comprises: a semiconductor layer functioning as an electron supplying layer; a surface layer disposed in spaced relation to the semiconductor layer, the surface layer being composed of a material having a negative electron affinity or a positive electron affinity close to zero; an electron transferring layer interposed between the semiconductor layer and the surface layer, the electron transferring layer supplying electrons in a direction from the semiconductor layer toward the surface layer; and a surface electrode disposed in Schottky contact with at least a part of the surface layer, the surface electrode applying a voltage such that the electrons are supplied from the semiconductor layer to an outermost surface of the surface layer.
By applying the voltage between the semiconductor layer and the surface electrode in Schottky contact with at least the part of the surface layer, the electrons can be supplied with excellent controllability from the semiconductor layer to the surface layer through the electron transferring layer. As a result, there can be obtained an efficient electron-emitting device with excellent controllability.
In the third electron-emitting device also, the electron transferring layer preferably has a varying composition such that the electron affinity decreases in the direction from the semiconductor layer toward the surface layer.
In the third electron-emitting device also, the electron transferring layer preferably has a varying composition such that a band gap is enlarged in the direction from the semiconductor layer toward the surface layer.
In the third electron-emitting device also, the electron transferring layer is preferably composed of a multilevel superlattice layer composed of a plurality of stacked layers for resonantly transmitting the electrons supplied in the direction from the semiconductor layer toward the surface layer.
In the third electron-emitting device also, the surface layer preferably contains aluminum nitride (AlN).
In the third electron-emitting device also, a region including the electron transferring layer and the surface layer is preferably composed of AlxGa1xe2x88x92xN (0xe2x89xa6xxe2x89xa61) in which the proportion of Al increases with approach toward the outermost surface.
In the third electron-emitting device, the surface electrode preferably has: a first region disposed in contact with only a part of the surface layer, the first region occupying an area smaller than the area occupied by the semiconductor layer; and a second region disposed continuously from the first region, an energy barrier between the surface layer and the second region being larger than an energy barrier between the surface layer and the first region.
In that case, the second region preferably occupies an area larger than the area occupied by the first region and is preferably composed of a material more endurable to ion bombardment than the material composing the first region. The arrangement increases the durability of the electron-emitting device.
Alternatively, the electron-emitting device further comprises an insulator layer formed on the region of the surface layer other than the region thereof in contact with the first region of the surface electrode, wherein the second region of the surface electrode is provided extensively over a side surface of the insulator layer and a top surface of the insulator layer. In the arrangement, the current of electrons can converge on the first region from which the electrons are emitted readily so that a more efficient electron-emitting device is implemented.
In the third electron-emitting device, the second region of the surface electrode preferably has a thickness larger than the thickness of the first region.
A fourth electron-emitting device of the present invention comprises: a solid layer functioning as an electron control layer; a surface layer disposed on the solid layer, the surface layer being composed of a material having a negative electron affinity or a positive electron affinity close to zero; a surface electrode disposed in contact with the surface layer, the surface electrode applying a voltage between the solid layer and an outermost surface of the surface layer; an external electrode disposed in spaced relation to the surface electrode; and a sealing member for maintaining a space between the surface electrode and the external electrode under reduced pressure.
The electrons emitted from the surface layer into a space under reduced pressure through the solid layer for controlling the quantity of emitted electrons by adjusting the value of the applied voltage are accelerated with the positive voltage applied to the external electrode to reach the external electrode. In other words, the kinetic energy of the emitted electrons can be controlled by adjusting the value of the voltage supplied to the external electrode. Since the quantity of emitted electrons can be controlled with the voltage applied between the solid layer and the surface electrode, the current flowing to the external electrode disposed in spaced relation to the surface electrode can eventually be controlled. By thus controlling the quantity of emitted electrons by adjusting the value of the voltage applied between the solid layer and the surface electrode, accelerating the electrons in the space under reduced pressure, and collecting the electrons by the external electrode, it becomes possible to cause the fourth electron-emitting device to function as an element (vacuum transistor) capable of amplifying a signal and performing a switching operation.
Since the device is composed of the solid/surface layers from which electrons are emitted easily and adapted to accelerate the emitted electrons under reduced pressure, it has the advantages of high withstand voltage, small internal loss, and low-voltage driving.
In the fourth electron-emitting device, the solid layer may have a structure provided at least with: a semiconductor layer functioning as an electron supplying layer; and a graded composition layer interposed between the semiconductor layer and the surface layer, the graded composition layer having a varying composition such that the electron affinity decreases in a direction from the semiconductor layer toward the surface layer.
In the fourth electron-emitting device, the graded composition layer preferably has a varying composition such that a band gap is enlarged in the direction from the semiconductor layer toward the surface layer.
In the fourth electron-emitting device, at least a part of the graded composition layer preferably has a band gap enlarged nearly continuously from the semiconductor layer to the surface layer.
In the fourth electron-emitting device, a region including the graded composition layer and the surface layer is preferably composed of AlxGa1xc3x97xN (0xe2x89xa6xxe2x89xa61) in which the proportion of Al increases with approach toward the outermost surface.
In the fourth electron-emitting device, the solid layer may have a structure provided at least with: a semiconductor layer functioning as an electron supplying layer; and a multilevel superlattice layer interposed between the semiconductor layer and the surface layer, the multilevel superlattice layer being composed of a plurality of stacked layers for resonantly transmitting electrons supplied in a direction from the semiconductor layer toward the surface layer.
In the fourth electron-emitting device, the surface layer preferably contains aluminum nitride (AlN).
In the fourth electron-emitting device, the surface electrode preferably has: a first region disposed in contact with only a part of the surface layer, the first region occupying an area smaller than the area occupied by the semiconductor layer; and a second region disposed continuously from the first region, an energy barrier between the surface layer and the second region being larger than an energy barrier between the surface layer and the first region.
In the fourth electron-emitting device, the second region preferably occupies an area larger than the area occupied by the first region and is preferably composed of a material more endurable to ion bombardment than the material composing the first region.
Preferably, the fourth electron-emitting device further comprises an insulator layer formed on the region of the surface layer other than the region thereof in contact with the first region of the surface electrode, wherein the second region of the surface electrode is provided extensively over a side surface of the insulator layer and a top surface of the insulator layer.